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Date: Tue, 25 Aug 2020 17:21:52 +0200 From: Sedat Dilek <sedat.dilek@...il.com> To: Arvind Sankar <nivedita@...m.mit.edu> Cc: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Segher Boessenkool <segher@...nel.crashing.org>, Thomas Gleixner <tglx@...utronix.de>, Nick Desaulniers <ndesaulniers@...gle.com>, "Paul E. McKenney" <paulmck@...nel.org>, Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Kees Cook <keescook@...omium.org>, Peter Zijlstra <peterz@...radead.org>, Juergen Gross <jgross@...e.com>, Andy Lutomirski <luto@...nel.org>, Andrew Cooper <andrew.cooper3@...rix.com>, LKML <linux-kernel@...r.kernel.org>, clang-built-linux <clang-built-linux@...glegroups.com>, Will Deacon <will@...nel.org> Subject: Re: [PATCH] x86/asm: Replace __force_order with memory clobber On Tue, Aug 25, 2020 at 5:19 PM Arvind Sankar <nivedita@...m.mit.edu> wrote: > > On Mon, Aug 24, 2020 at 09:13:34PM +0200, Miguel Ojeda wrote: > > Hi Arvind, > > > > On Sun, Aug 23, 2020 at 11:25 PM Arvind Sankar <nivedita@...m.mit.edu> wrote: > > > > > > - Using a dummy input operand with an arbitrary constant address for the > > > read functions, instead of a global variable. This will prevent reads > > > from being reordered across writes, while allowing memory loads to be > > > cached/reordered across CRn reads, which should be safe. > > > > Assuming no surprises from compilers, this looks better than dealing > > with different code for each compiler. > > > > > Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu> > > > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602 > > > > A lore link to the other discussion would be nice here for context. > > > > Ok. > > > > + * The compiler should not reorder volatile asm, however older versions of GCC > > > + * had a bug (which was fixed in 8.1, 7.3 and 6.5) where they could sometimes > > > > I'd mention the state of GCC 5 here. > > > > Ok. > > > > + * reorder volatile asm. The write functions are not a problem since they have > > > + * memory clobbers preventing reordering. To prevent reads from being reordered > > > + * with respect to writes, use a dummy memory operand. > > > */ > > > -extern unsigned long __force_order; > > > + > > > > Spurious newline? > > > > This was intentional, but I can remove it if people don't like the extra > whitespace. > > I'll wait a few days for additional review comments before sending v2. > > Thanks. Thanks for taking care and your patch. I have tested this on top of Linux v5.9-rc2 with LLVM toolchain v11.0.0-rc2 (ThinLTO). Tested-by: Sedat Dilek <sedat.dilek@...il.com> - Sedat -
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